A fuel cell has been proposed as a clean, efficient and environmentally responsible energy source for various applications. Individual fuel cells can be stacked together in series to form a fuel cell stack. The fuel cell stack is capable of supplying a quantity of electricity sufficient to provide power to an electric vehicle. In particular, the fuel cell stack has been identified as a desirable alternative for the traditional internal-combustion engine used in modern vehicles.
One type of fuel cell stack is known as a proton exchange membrane (PEM) fuel cell stack. The typical PEM fuel cell includes three basic components: a cathode, an anode, and an electrolyte membrane. The cathode and anode typically include a finely divided catalyst, such as platinum, supported on carbon particles and mixed with an ionomer. The electrolyte membrane is sandwiched between the cathode and the anode. Porous diffusion media which facilitate a delivery and distribution of reactants, such as hydrogen gas and air, may be disposed adjacent the anode and the cathode.
In a vehicle power system employing the PEM fuel cell stack, the hydrogen gas is supplied to the anodes from a hydrogen storage source, such as a pressurized hydrogen tank. The air is supplied to the cathodes by an air compressor unit. The hydrogen gas reacts electrochemically in the presence of the anode to produce electrons and protons. The electrons are conducted from the anode to the cathode through an electrical circuit disposed therebetween. The protons pass through the electrolyte membrane to the cathode where oxygen from the air reacts electrochemically to produce oxygen anions. The oxygen anions react with the protons to form water as a reaction product.
The electrochemical fuel cell reaction also has a known temperature range within which the reaction may efficiently occur. The electrochemical fuel cell reaction is exothermic and generally allows the fuel cell stack to maintain a temperature within the desired temperature range during an operation thereof. Supplemental heating is typically employed during a start-up operation of the fuel cell stack to raise the temperature of the fuel cell stack within the desired temperature range. For example, the fuel cell stack may be in fluid communication with a coolant system that circulates a coolant through the fuel cell stack. The coolant may be heated, such as with electrical heaters, to raise the temperature of the fuel cell stack. The coolant may also transfer excess heat away from the fuel cell stack by circulating through a radiator that exhausts the heat to the ambient atmosphere.
It is known to regulate the temperature of the fuel cell stack by diverting coolant around the radiator when a heating of the fuel cell stack is desired, and by directing coolant to the radiator when a cooling of the fuel cell stack is desired. Diverter assemblies or valves that selectively modify the coolant flow are employed as thermostats within the fuel cell system. Known diverter valves include rotating disc-type valves, three-way ball valves, three-way plug valves, and three-way butterfly valves. The rotating disc-type valves, three-way ball valves, and three-way plug valves have sliding seals that permit leaking between the valve seated positions, and may also require an undesirable amount of torque to actuate.
Three-way butterfly valves have a substantially flat plate positioned inside the valve body. The flat plate is coupled to a rod that turns the plate to positional limits parallel or perpendicular the coolant flow. The flat plate is restrictive to the coolant flow when rotated to either end of the valve positional limits. Three-way butterfly valves are also able to be actuated with a more desirable amount of torque than with the other known valves. However, since the flat plate is always present within the flow, regardless of position, an undesirable pressure drop across the valve is often induced. Three-way butterfly valves are also known to exhibit undesirable flow control between the valve positional limits. The flow control between the valve positional limits in conventional three-way butterfly valves is known to be substantially and undesirably nonlinear.
There is a continuing need for a diverter assembly that exhibits greater flow controllability, improved pressure drop characteristics, and a desirable torque actuation requirement than valves known in the art. Desirably, the diverter assembly may be employed as a thermostat in a fuel cell system.